1. Field of the Invention
The present invention relates to the preparation and use of modified zeolite catalyst compositions which are especially suitable for the alkylation of aromatic hydrocarbons to provide product mixtures enriched in the para-(or 1,4-)dialkyl substituted benzene isomer.
2. Description of the Prior Art
Production of dialkyl substituted benzene compounds via alkylation of aromatic hydrocarbons is an important step in a number of commercial chemical manufacturing processes. Such reactions can be carried out over a variety of catalyst materials. Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has, for example, been described. U.S. Pat. No. 2,904,697 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
In many of these prior art processes, the dialkylbenzene product produced frequently contains more of the 1,3 isomer than of the other two isomers. For example, xylene produced via the conventional catalytic methylation of toluene can have the equilibrium composition of approximately 24 percent of 1,4-, 54 percent of 1,3- and 22 percent of 1,2-isomer. Of the dialkylbenzene isomers, 1,3-dialkylbenzene is often the least desired product, with 1,2- and 1,4-dialkylbenzene being the more useful products. 1,4-Dimethylbenzene, for example, is of particular value, being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron". Furthermore, 1,4-methylethylbenzene, i.e., para-ethyltoluene (PET), is useful for subsequent conversion to para-methylstyrene, and for this purpose ethyltoluene products containing as much as 97% of the para isomer can be required.
Mixtures of dialkylbenzene isomers, either alone or in further admixture with ethylbenzene, have previously been separated by expensive superfractionation and multistage refrigeration steps. Such processes, as will be realized, involve high operation costs and have a limited yield. Alternatively, various modified zeolite catalysts have been developed to alkylate toluene with a greater or lesser degree of selectivity to 1,4-dialkylbenzene isomers. Hence, U.S. Pat. Nos. 3,972,832, 4,034,053, 4,128,592, and 4,137,195 disclose particular zeolite catalysts which have been treated with compounds of phosphorus and/or magnesium to increase para-selectivity of the catalysts. Para-selective boron-containing zeolites are shown in U.S. Pat. No. 4,067,920 and para-selective, antimony-containing zeolites in U.S. Pat. No. 3,979,472. Similarly, U.S. Pat. Nos. 3,965,208; 4,117,026; 4,259,537; 4,260,843; 4,275,256; 4,276,437; 4,276,438; 4,278,827 and 4,288,647 all disclose other zeolites modified with various oxides to improve catalyst para-selectivity.
Notwithstanding the existence of such chemically-modified zeolite catalysts, methods have also been developed to treat such catalysts in an effort to enhance their para-selectivity properties even further. Thus, for example, European Patent Application No. 80304118.5, published June 24, 1981, discloses a process whereby para-selectivity of oxide modified ZSM-5 type zeolite catalysts can be improved by treating such selectivated catalysts with a carboxylic acid such as acetic acid. While such procedures are effective in enhancing catalyst selectivity, catalyst treatment with such materials may sometimes deactivate the catalyst and result in a decrease in conversion when such selectivated catalysts are employed in aromatics alkylation reactions. There is thus a continuing need to develop catalyst preparation procedures which provide highly para-selective zeolite-based catalysts but which do not significantly deactivate the catalyst as a consequence of improved catalyst selectivity.
Accordingly, it is an object of the present invention to provide modified zeolite catalyst compositions which promote the alkylation of aromatics to produce mixtures containing an exceptionally high percentage, e.g., 80% by weight or more for alkylation to toluene, of para-dialkylbenzene isomer.
It is a further object of the present invention to provide methods of preparing such highly para-selective catalysts without significantly reducing catalyst activity.
It is a further object of the present invention to provide highly para-selective alkylation processes employing the modified zeolite catalysts described herein.